The present invention is directed generally to systems and methods for implementing a fan-based thermal management system.
Fan-based thermal management systems for dissipating excess heat generated by the circuitry, or “payload,” of a host system are well known in the electronic arts. The payload may be, for example, a computer microprocessor and its associated components. For host systems having relatively small thermal loads, constant-speed fans provide an attractive solution in terms of simplicity and low cost. Although such fans are typically sized so that the cooling effect provided exceeds that actually required, the cost of this excess capacity is generally small compared with overall operation costs.
The use of constant-speed fans may not be desirable, however, for host systems in which the thermal load is significant, such as, for example, an enclosure containing multiple servers. The power necessary to operate cooling fans in such applications may represent an appreciable portion of the overall operation costs. Accordingly, eliminating excess thermal cooling capacity by adjusting fan speed to optimally match actual cooling requirements reduces power consumption, and thus operation costs. Additionally, where numerous thermal cooling fans are utilized in close physical proximity to each other, the acoustic noise due to fan operation may be problematic. Adjusting fan to speed to provide optimal cooling thus has the further benefit of quieter system operation.
The robustness of a thermal management system is determined largely by its ability to maintain the temperature stability of the payload over a desired range of ambient temperature. Variable-speed fan control is commonly implemented using a digital controller that is programmed to maintain system temperature at a fixed temperature setpoint. For example, the controller may first measure current fan speed using a tachometer feedback signal. The controller may next measure the system temperature using a temperature sensor. If the controller determines that the system temperature exceeds the temperature setpoint, the controller may increase fan speed slightly, increasing airflow and thus causing the system temperature to decrease. Conversely, if the system temperature is less than the temperature setpoint, the controller may decrease fan speed slightly, reducing airflow and thus allowing the system temperature to increase. In order to determine the amount of speed adjustment necessary, the controller may calculate a new fan speed output based on the temperature error (i.e., the difference between the current system temperature and the temperature setpoint) and the current speed output. The calculated speed output and the measured fan speed are then compared to determine the actual increase or decrease in fan speed required. These steps may be repeated continuously, with a sufficient time delay introduced between iterations to allow the system temperature to sufficiently react to airflow changes. Stable control is achieved when the controller is able to maintain the system temperature about the setpoint with little or no fluctuation.
In a thermal management system implementing the above-described fixed-setpoint control scheme, the ideal speed versus ambient temperature (“speed-temperature”) control response would specify a minimum fan speed at or below the lower limit of the ambient temperature range and linearly ramp up a maximum speed at or above the upper limit. For a push-through configuration in which the temperature sensor is located upstream with respect to the payload, the fixed-setpoint controller output can be made to approximate this ideal response using empirical calibration techniques. Use of a fixed setpoint-control scheme in a push-through configuration may still result in significant temperature fluctuations in the downstream payload, however, due to the upstream location of the temperature sensor.
A more robust control thermal management system may be realized through the use of a pull-through configuration in which the temperature sensor is positioned downstream with respect to the payload. Because the measured temperature is a function of both the ambient and payload temperatures, a pull-through configuration would make it possible to better maintain the temperature stability of the payload. However, calibrating the controller to approximate the ideal control response in a pull-through configuration is problematic. In particular, implementing a controller based on the fixed-setpoint design of a push-through configuration results in an unstable controller output that saturates prematurely in response to small temperature changes. Additionally, maximum fan speed occurs at an ambient temperature significantly lower than that specified by the ideal speed-temperature control response. These problems are largely attributable to the constant gain of the fixed-setpoint control scheme.
Accordingly, there exists a need for a system and method for realizing stable fan speed control in a thermal management system having a pull-through configuration.